ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

After testing, the properties of passive soft tissue structures were investigated by
performing a resection study using a stepwise resection of the passive soft tissue
structures [9].
3.2 Virtual Model Creation
For each tested specimen, a specimen specific computer model was built in
Abaqus/Explicit (Dassault Systèmes Simulia Corp., Providence, RI, USA). A model
consisted of the 3D geometry of the femur, tibia and fibula bones including the joint
cartilage based on CT and MRI scans (Figure 1A). The patella was not included. Virtual
TKR surgery was performed using the same prostheses type, size and positioning
parameters as performed during experiments (Figure 1B, C).
A
Figure 1: A) 3D geometry of virtual model, B) with implanted femur and tibia component, C) with
meshed implant articulating surfaces and visualized rigid body reference nodes.
The prosthesis component articulating surfaces were meshed using quad dominated
meshes of shell elements with global average element edge length of 1.8 mm for the
femur component and 1.5 mm for the tibia component (insert). Both meshed bodies
were defined to be rigid bodies. A rigid body analysis allows determining relative
kinematics with reduced computational time compared to a deformable analysis.
Contact between the joint surfaces was defined using the penalty based method with a
weight factor. The contact forces are defined as a function of the overclosure
(penetration) of the master (femoral component) into the slave surface. This “bed of
springs” method [10] from the elastic foundation (EF) theory [1,11,12] used the
pressure overclosure (PO) relationship [10]:
pressure
B
1
E
overclosur
1
1 2
thickness
Femur bone
Femur component
Tibia component with
insert
Tibia bone
Fibula bone
Equation 1 [10]
e
where E is the Young’s modulus of the insert material and is the Poisson’s ratio. In
order to improve the estimates of a linear relationship a piecewise linear pressure PO
curve was estimated based on a nonlinear stress strain material curve of the insert
material polyethylene (PE) [10].
The insert height was dependent on the implanted type of insert used during the robot
experiments. For each model the insert thickness was used as input parameter for the
PO relationship. In tangential direction, frictional contact was defined using a friction
coefficient as a function of sliding velocity and contact pressure determined
experimentally [13].
To drive the model boundary conditions were applied to the rigid body reference nodes.
Consistent with the experimental protocol, the femur bone reference node of the knee
C
Reference
nodes